The present invention relates to methods of preparing asymmetric membranes for the separation of components from fluid mixtures and solutions.
Asymmetric membranes, often referred to as anisotropic membranes, are membranes with a very thin, dense "active" surface supported by a porous substructure. They are useful for separating a wide variety of impurities and components from solutions. For example, asymmetric membranes have been developed and used as reverse osmosis membranes in the desalination of brackish and saline waters. They have also been developed for the separation of gases, such as the separation of acid gases from methane's as described in U.S. Pat. Nos. 3,534,528: 4,130,403 and 4,561,654, or the separation of other gas mixtures as described in U.S. Pat. No.4,230,463.
Asymmetric membranes, such as the cellulose acetate membranes known in the art, are conventionally prepared using a solution casting process. In this process a polymer is dissolved in a mixture of solvents of diverse volatilities, swelling agents and pore forming substances. The solution is cast into a thin film and the more volatile solvents are allowed to partially evaporate. The film is then immersed in a nonsolvent of the polymer to precipitate the polymer and to thereby form the asymmetric structure.
Solution casting requires careful selection and control of process parameters. These parameters include the selection of the casting solvents and additives, the temperature of the casting solution, and the method of solvent removal. Unfortunately, polymers possessing superior chemical, mechanical and thermal properties are often difficult to fabricate because of these process limitations. For example, the selection of a solvent and nonsolvent for a specific polymer often requires laborious trial and error.
In view of the deficiencies of the prior art, it is desirable to provide a method for preparing asymmetric membranes that does not entail onerous process limitations. It is also desirable to provide a method for preparing asymmetric membranes exhibiting good permeation flux and selectivity.